1. Field of the Invention
Embodiments of the invention relate to a semiconductor device such as a vertical MOSFET that uses a wide bandgap semiconductor material and has a trench structure.
2. Description of the Related Art
In a vertical MOSFET, the cell density per unit surface area may be increased to a greater extent with a trench structure in which the channel is formed orthogonally with respect to the substrate surface as compared to a planar structure in which the channel is formed parallel to the substrate surface and therefore, from the perspective of cost, is advantageous.
In a trench structure, as the angle of the sidewall of the trench approaches perpendicularity to a main surface, channel mobility increases, and increased current density and decreased ON resistance are facilitated. On the other hand, when the trench is formed by plasma dry etching to achieve a sidewall that is nearly perpendicular, the curvature of the corners of the bottom of the trench decreases, whereby a problem arises in that breakdown voltage decreases due to electric field concentration.
When trench formation conditions are set so that the curvature of the bottom of the trench is large, the angle of the sidewall of the trench tends to be obtuse, leading to a problem of decreased mobility. Further, when the angle of the sidewall of the trench is nearly perpendicular and a deposited oxide film is used for a gate oxide film, the thickness of the deposited gate oxide film decreases with proximity to the bottom of the trench and the gate oxide film is easily destroyed at locations where the gate oxide film is thin.
Conventionally, a structure has been proposed in which the angle of the sidewall of a trench part is sloped in 2 stages (for example, refer to Japanese Laid-Open Patent Publication No. 2014-56882).